1. Field of the Invention
The present invention relates to seismic survey and exploration, and to displays representing the results thereof.
2. Description of Prior Art
A typical seismic survey for hydrocarbons is conducted as a series of intersecting profiles along seismic survey lines on a rectangular grid. Each such profile yields a seismic reflection section, which displays, generally to a first approximation, the configuration of the rock interfaces under the line of profile.
In the traditional practice of seismic interpretation, the first operation is to select, or "pick", as the process is known in the art, the reflections which represent these interfaces. This is a process involving considerable judgment, based on knowledge of the regional and local geology. Often the form of a reflection on one or more sections is obscure, and closure of the pick around a loop of the grid is necessary to resolve it. It is therefore common practice to pick each of several reflectors and to check all loop ties before proceeding further.
The next operation is to "time" each of the picked reflections at points spaced at suitable intervals along the surface, and to "post" these times, for each reflector, on maps of the profiles. Suitable contour intervals are then interpolated between the posted times, and a contour map is then constructed for each picked reflector. Perspective views of the contoured surface may then be generated, and numerical integrations may be made to assess volumes.
In the current practice of the art, the timing, posting, contouring and later operations are mechanized using a digitizer, a computer and a computer-controlled plotter. The picking operation, however, remains one which must be done by a skilled person. Decisions must be made on the geological likelihood of one possible interpretation against another, geological faults must be identified by visual character correlation, and a knowledge of the general geological history of the area must be introduced into the picking process. These matters are often very problematical, and their resolution very tedious; each possible interpretation of a difficult segment on one section requires the closing of a loop involving other sections, and each of the latter sections may raise its own ambiguities of interpretation. Furthermore, each reference to another section represents a cumbersome and time-consuming operation, so that the sheer volume of work tends to limit the number of interpretive iterations which can be made.
These difficulties are itensified by the general problem of visualizing three-dimensional subsurface features of the earth from two-dimensional seismic sections. An additional weakness of the traditional practice is that many subtleties which may be present on the seismic sections themselves, and which subtleties may be important to the search for hydrocarbons, are smoothed out in the picking so that their existence is not apparent on the contour map.